What did moviemakers do before The Seven Samurai was made? Surely that film didn’t invent the “hastily assembled bunch of fighters has to defend a place against impossible odds” scenario but it feels like it did, and it’s inspired countless imitators of which Ironclad is the latest.

It’s England in the year 1215, and King John (Paul Giamatti) has been such a lousy king that his barons have rebelled and forced him to sign the Magna Carta, in which he promises to be not such a lousy king. As you can imagine, the king wasn’t real thrilled about this and he’s decided to teach the barons a lesson by systematically taking over their castles and slaughtering the inhabitants, aided and abetted by a group of Danish mercenaries (led by Vladimir Kulich, aka “the guy who was Buliwyf in The Thirteenth Warrior”). John’s next target is Rochester castle, which will give him control of all of southern England.

Luckily, the duke of Albany (Brian Cox) has other ideas. He teams up with a disillusioned Templar Knight (James Purefoy) and a ragtag bunch of fighters to hold the castle until French allies arrive to help out the rebellion against John. Then follows a lot of fighting and sieges, but not as much drama or excitement as you’d think.

Ironclad has a lot going for it. The cast alone makes it worth a rental. Purefoy does a great deal with a character that’s thinly sketched to say the least (more on that in a moment), conveying with few words the toll that years of fighting has taken on his soul. Cox is charismatic and the screen lights up whenever he’s on it. Giamatti seems miscast at first, but in the film’s second half he gets in touch with his inner despot and portrays John as a sociopathic little man who will do anything to maintain absolute power. And though he only gets three scenes, Charles Dance is on hand to lend some dignity and authority to matters as the archbishop of Canterbury, who’s willing to risk all to keep the rebellion going.

Not all the cast works, though. Derek Jacobi is a bit hammy as lord of Rochester castle, and Kate Mara as his unhappy, sexually frustrated wife is just terrible. Mara has a nice medieval look to her, but her character is poorly conceived – she’s supposed to be Purefoy’s soul mate, but comes off as petulant instead – and unlike most of the cast, Mara isn’t a strong enough actor to make the role work.

Which leads us to the major flaw of Ironclad – the characterization is so thin as to be transparent. The actors have to let their talent and charisma do the heavy lifting, because the screenplay does them no favors. Particularly ill-served are the ragtags that Albany recruits – I never remembered their names. They were “the archer dude” and “the guy who’s kind of an asshole” and “the bearded stocky guy who looks like that other bearded stocky guy” and “the guy who looks like Frodo Baggins”. When the stakes are this high for the characters, you should be able to care about them. But sympathy for the characters ranks far below that generated by the actors.

What Ironclad does have is nasty battles, and a lot of it. The movie has a gritty, gloomy feel to it, and makes it clear that medieval life in general, and warfare in particular, were no picnic. I appreciated the attention to detail (i.e., when preparing for the siege, the Templar makes sure to find out where the castle’s water supply comes from and how at risk it is). Director Jonathan English is a little too fond of the spinny-shaky cam, but the action looks good and (to my amateur’s eyes) reasonably authentic. It’s also very gory, with limbs and hands a-flying and several memorable scenes of people being cloven in twain.

Yet here’s something lifeless about the whole affair. A stronger screenplay and better characterization would have made this a classic, but as it stands Ironclad is serviceable.